ABSTRACT Autoimmune diseases affect ~ 5% of the U.S. population and carry a high burden of morbidity, health care costs, disability, and premature mortality. In the first cycle of this award, we leveraged an innovative nationwide NIH-funded double-blind, placebo-controlled randomized trial, VITAL, to test the effects of vitamin D (vitamin D3 [cholecalciferol]) and marine omega-3 fatty acid (eicosapentaenoic acid [EPA] + docosahexaenoic acid [DHA]) supplements upon the risk of incident autoimmune disease and changes in biomarkers of systemic inflammation. Data from laboratory studies, observational epidemiologic research, and small prevention trials strongly suggest that these nutritional agents have anti-inflammatory and immunomodulating benefits. Popular enthusiasm for vitamin D and fish oil supplements underscores the urgent need for rigorous testing. We have recruited, randomized, and are following 25,874 VITAL participants, men aged ?50 and women aged ?55 nationwide, including 20% African Americans. Following a 3 month run-in, eligible participants were randomly assigned to one of four treatment groups: vitamin D3 (2000 IU/d) and fish oil (EPA+DHA, 1 g/d); vitamin D3 and fish oil placebo; placebo vitamin D3 and fish oil; and placebo vitamin D3 and placebo fish oil. At yearly intervals, all participants receive a new pill supply, are asked about compliance and side effects, and report incident autoimmune diseases. A physician endpoints committee has confirmed 444 incident autoimmune disease cases by medical record review to date. In a randomly selected subcohort of 1634 VITAL participants, blood samples have been assayed for changes in C-reactive peptide, interleukin-6, and tumor necrosis factor- receptor 2 in all four trial arms. With this renewal grant, we will complete the 5 pre-specified years and a 2 year observational extension, critically important given the long latency of autoimmune disease onset. Continued follow-up will improve statistical power for detecting preventive effects on autoimmune disease incidence, and will enable investigations of effects over time and effect modification by baseline factors and biomarkers. We hypothesize that there will be a delayed reduction in autoimmune disease, and that the largest preventive effects will be among those with high systemic inflammation, including the obese and those with elevated baseline biomarkers of inflammation. In this renewal, we also will test for changes in ?Specialized Pro- Resolving Mediators? (SPM), novel omega-3 fatty acid-dependent lipids responsible for inflammation resolution. We will employ cutting-edge quantitative liquid chromatography-tandem mass spectroscopy to extend understanding of the biological mechanisms by which omega-3 fatty acids influence inflammation resolution and potentially autoimmune disease pathogenesis. Given the ongoing NIH-funded VITAL trial infrastructure, our strong multidisciplinary research team, and success with prior large mail-based trials and cohort studies, with continued funding these investigations will furnish robust and definitive results with important public health ramifications.